Coatings deposited on metallic substrates are extensively used in consumer and industrial applications. The most commonly used industrial coating is Cr (Cr) which is electrodeposited from its hexavalent state from aqueous electrolytes. Cr coatings (0.00025″ to 0.010″ thick) are used extensively for imparting wear and erosion resistance to components in industrial, aerospace and military applications because of their intrinsic high hardness (600-1,000 VHN) and their low coefficient of friction (<0.2). Hard Cr electrodeposition from hexavalent Cr baths is used to apply hard Cr coatings to a variety of aircraft components in manufacturing and repair and overhaul operations, most notably landing gear, hydraulic actuators, gas turbine engines, helicopter dynamic components and propeller hubs. Process and performance drawbacks of Cr coatings include the low current efficiency of the hexavalent Cr plating processes, low deposition (or build) rates compared to the plating of other metals and alloys (e.g., 12.5 μm to 25 μm per hour for Cr versus over 200 μm per hour for Ni). The intrinsic stress and brittleness of Cr deposits (i.e., <0.1% tensile elongation) invariably leads to micro- or macro-cracked deposits. Although these ‘cracks’ do not compromise wear and erosion resistance, cracked or porous coatings are unacceptable for applications requiring corrosion and/or fatigue resistance. Voids, macro and micro cracks in coatings allow for moisture ingress severely limiting the corrosion resistance of e.g., Cr plated steel parts. Further, these defects act as stress concentrators and invariably become crack initiation sites under low stress (i.e., <yield stress) cyclic loading, thereby reducing the fatigue life of the component (i.e., imparting a fatigue debit). In corrosion protection applications, an electrodeposited under layer of more ductile and corrosion resistant material (usually Ni) must be applied, or the substrate must be shot-peened to impart a residual compressive stress to improve the fatigue resistance.
The most common health effect from exposure to Cr metal is contact dermatitis, a skin inflammation or rash. A fraction of the population, between 5 and 10 percent, has an allergic skin reaction to Cr which, much like other allergies, is genetically based. Avoiding skin contact with Cr—in jewelry for example—is not a problem for most of the general population but it is for those whose occupations involve daily exposure to Cr compounds, such as, e.g., cement workers and electroplaters, which may develop chronic allergic reactions.
As a result of the toxicity of Cr compounds, maximum exposure levels of chromate ions are regulated. The US Department of Labor's Occupational Safety and Health Administration (OSHA) recently reduced the permissible exposure limit (PEL) for hexavalent Cr and all hexavalent Cr compounds from 52 μg/m3 to 5 μg/m3 as an 8-hour time weighted average. In addition to tighter limits on air pollution the EPA has also set new limits for Cr in the water recognizing that the electrodeposition of Cr is a hazardous process. Due to the expected increase in operational costs associated with compliance to the proposed rule, environmentally benign alternatives to hard Cr plating are being sought. As well the EPA has listed Ni as a pollutant which is to be phased out.
It is well documented that applying electroplated coatings including Ni and Cr to steel reduces the fatigue performance of the plated part.
From the aforementioned, it is apparent that there is great need to replace electroplated Cr coatings with Cr- and Ni-free wear resistant coatings which meet or exceed the physical properties of Cr coatings with alternative coatings which are biocompatible, safe, are not limited to line-of sight applications and introduce properties not inherent to Cr based coatings, including, but not limited to, low porosity, enhanced fatigue resistance, non-wetting and anti-microbial properties.
Coating technologies considered as suitable Cr alternatives include other suitable Cr-free coatings applied by electrolytic or electroless plating techniques as well as thermal spray processes including high velocity oxygen fuel (HVOF) thermal spray and plasma vapor deposition. Although HVOF thermal spray coatings generally meet the properties of electrodeposited Cr, their application is limited to line-of-sight applications, i.e., the inside diameter of tubular structures less than 40 cm in diameter and 1:1 width-to-depth aspect ratios and blind holes cannot be coated using this technology.
For coating applications requiring non-line-of-sight deposition and/or high-volume, low-added-value production, it is generally accepted that only electroplating technologies will be suitable and/or cost effective. Traditionally, most of the electroplated coating alternatives have been based on Ni alloys, including both electroless (Ni—P and Ni—B) and electrolytic (Ni—W, Ni—Co, Ni—Mo, etc.) coatings. As Ni is listed by the Environmental Protection Agency (EPA) as a priority pollutant and is considered to be one of the 14 most toxic heavy metals, coatings containing Ni, at best, are considered to represent a short-term solution. Bath stability issues and adhesion failures limit the use of electroless coatings particularly in aerospace applications.
It is therefore evident that a Ni-free electroplating technology would be ideal to provide an environmentally acceptable alternative for non-line-of-sight applications currently addressed with Cr.
The prior art has disclosed the use of cobalt (Co) bearing electrodeposited coatings:
Brenner in U.S. Pat. No. 2,643,221 (1950) discloses the electrodeposition of Ni—P and Co—P alloy coatings from solutions containing the metal ions and phosphates and considered them suitable for use as alternatives to Cr electrodeposits. Specifically to Co-bearing coatings, Brenner noted that they were dull at lower than 9% phosphorus (P), they turned black when exposed to oxidizing conditions and overall Brenner prefers Ni—P for protective and decorative applications. Brenner is silent on the microstructure, the coating stress, the fatigue performance, the wetting behavior and the antibacterial properties of all coatings.
Holko in U.S. Pat. No. 5,358,547 (1994) and U.S. Pat. No. 5,649,994 (1997) describes wear resistant coatings of cobalt and phosphorus for metallic surfaces. The preferred composition consists of Co-11P, i.e., 11% per weight of P, which represents approximately the “eutectic” composition. Preferred applications include surgical blades, files and burrs, guide slots, drills and drill guides, surgical instruments and medical prostheses. While Holko's preferred method of application is the use of metal powders and binders followed by heat-treatment, i.e., powder metallurgy, as illustrated in all examples, Holko does extend his application methods to include other synthesis processes such as electroless and electrodeposition. Holko is silent on the microstructure, the coating stress, the fatigue performance, the wetting behavior and the antibacterial properties of the coatings.
Tang in U.S. Pat. No. 6,036,833 (2000) discloses electroplated nickel, cobalt, nickel alloys or cobalt alloys without any internal stress deposited from a Watts bath, a chloride bath or a combination thereof, by employing pulse plating with periodic reverse pulses and sulfonated naphthalene additives.
Engelhaupt in U.S. Pat. No. 6,406,611 (2002) describes amorphous electrodeposited Ni—P or Ni—Co—P alloys which are essentially free of stress.
Ware in US 2005/0170201 and US 2007/0084731 describes coarse-grained Co—P—B coatings of low compressive residual stress and improved fatigue resistance. Ware discusses the “nanophase Co alloy coating” developed by Integran Technologies Inc. of Toronto, Canada, the applicant of this case. According to Ware this technology requires plating equipment that is different from the existing Cr plating equipment and, therefore, requires costly modifications of existing facilities. Ware states that high residual stress of Co alloy coatings results in an unacceptable decrease in fatigue resistance.
As highlighted by Ware, electrodeposited nanocrystalline Co based electrodeposited coatings have been proposed by Integran Technologies Inc. of Toronto, Canada, the applicant of this invention, as an alternative to electrodeposited Cr coatings.
Erb in U.S. Pat. No. 5,352,266 (1994), and U.S. Pat. No. 5,433,797 (1995), assigned to the applicant, describe a process for producing nanocrystalline materials, particularly nanocrystalline nickel. The nanocrystalline material is electrodeposited onto the cathode in an aqueous acidic electrolytic cell by application of a pulsed current.
Palumbo in US 2005/0205425 and DE 10,228,323 (2005), assigned to the same applicant, discloses a process for forming coatings or freestanding deposits of nanocrystalline metals, metal alloys or metal matrix composites. The process employs tank, drum plating or selective plating processes using aqueous electrolytes and optionally a non-stationary anode or cathode. Nanocrystalline metal matrix composites are disclosed as well.
Tomantschger in US 2009/0159451, assigned to the same applicant, discloses variable property deposits of fine-grained and amorphous metallic materials, optionally containing solid particulates.
Palumbo in U.S. Pat. No. 7,320,832 (2008), assigned to the same applicant, discloses means for matching the coefficient of thermal expansion of a fine-grained metallic coating to the one of the substrate by adjusting the composition of the alloy and/or by varying the chemistry and volume fraction of particulates embedded in the coating. The fine-grained metallic coatings are particularly suited for strong and lightweight articles, precision molds, sporting goods, automotive parts and components exposed to thermal cycling. Maintaining low coefficients of thermal expansion and matching the coefficient of thermal expansion of the fine-grained metallic coating with the one of the substrate minimizes dimensional changes during thermal cycling and prevents delamination.
The prior art describes numerous processes for affecting fatigue performance and to deal with the loss of fatigue resistance (fatigue debit) imparted by electrodeposited coatings.
Nascimento et. al. in the International Journal of Fatigue 23 (2001), 607-618, reports various fatigue data for surface treated and untreated 4340 aeronautical steel for electroplated Cr and electroless Ni coatings. In all instances, the addition of a coating layer showed a decrease in fatigue strength. All Cr-containing coatings resulted in poorer fatigue performance than the uncoated material as evident in FIGS. 2, 6, 9, 11 and 14.
Sriraman et. al. in Materials Letters 61 (2007) 715-718 reports on the fatigue resistance of steel coated with nanocrystalline Ni—W alloys by electrodeposition and reports inferior fatigue lives for all coated samples. Greenfield in U.S. Pat. No. 4,168,183 (1979) describes a process for improving the fatigue properties by coating the substrate with materials that contain a solute, prestraining the part to create dislocations in the surface layer, and annealing to diffuse the solute into the deformed surface layer.
The prior art has also disclosed the use of metals for use in anti-microbial and anti-bacterial applications:
Burrell in U.S. Pat. No. 5,681,575 (1997) and U.S. Pat. No. 5,753,251 (1998) teaches the synthesis of antimicrobial metals, specifically Ag, Cu, Sn, Zn and noble metals, which release ions exhibiting enhanced antimicrobial activity that is intrinsic to the bulk metal by virtue of its high stored internal energy. Note that Burrell's definition of “metals” is not limited to what is generally accepted to represent “metallic materials”, i.e., metals and alloys, but is significantly expanded to also include electrically non-conductive metal compounds such as oxides, nitrides, borides, sulfides, halides and hydrides. The optimized, sustained ionic dissolution rate is due to the ultrafine-grained microstructure of the “metallic films”. While it is demonstrated that a distinctly enhanced, sustained anti-microbial effect is associated with the processing of “metals” in fine-grained form, the material processing technique of Burrell et al, is based upon vapor deposition methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). While such techniques are suitable for the synthesis of fine-grained anti-microbial materials, unfortunately they are not suited for the production of highly abrasive, wear-resistant, scratch-resistant and scuff-resistant surfaces as the vapor deposited coatings are generally thin (typically <<10 μm thickness), porous (<<98% theoretical density) and soft (<200 VHN).
In order to satisfy the basic durability requirements of hospital, household, and consumer goods touch-surfaces, especially in high traffic areas, the inherent mechanical limitations of thin and porous sputtered antimicrobial films as disclosed in the Burrel patents must be overcome. This necessitates the use of a processing technique capable of producing fine-grained metallic materials that exhibit the desired unassisted sustained release of metal ions inherent to fine-grained microstructures while simultaneously exhibiting good hardness, strength, toughness, scratch resistance, abrasive/sliding wear, and scuff resistance properties.
The prior art also describes various means of increasing the water repellent properties of hydrophobic, predominantly polymeric surfaces by surface roughening.
Dettre in U.S. Pat. No. 3,354,022 (1967) describes water repellent surfaces, having an intrinsic advancing water contact angle of more than 90° and an intrinsic receding water contact angle of at least 75°, by creating a micro rough structure with elevations and depressions in a hydrophobic material. The high and low portions have an average distance of not more than 1,000 microns. The average height of high portions is at least 0.5 times the average distance between them. The air content is at least 60% and, in particular, fluorine containing polymers are disclosed as the hydrophobic material of choice.